A proposed molecular memory cell that would allow laptop computer batteries to last 100 times longer than today’s batteries is being modeled computationally on an IBM supercomputer at ORNL. This machine is also being used to simulate electron transport in carbon nanotubes in contact with other components, for future nanoscale electronic devices.

Designing
Electronic Devices Using Supercomputers

A large supercomputer at ORNL
is being used to learn more about the best ways to design electronic device
components on a very small scale.

Designing Nanocircuits

The successor to the silicon chip may be a nanoscale
devicea self-assembled monolayer of organic molecules of benzene
(a ring of six carbon atoms bonded with four hydrogen atoms) attached
to sulfur atoms at each end. Because sulfur (thiol) has an affinity for
gold, a single layer of these benzenedithiol molecules can be sandwiched
between thin gold contacts. Scientists at Rice and Yale universities have
induced self-assembly of such a device by dipping a gold surface into
a beaker of benzenedithiol molecules. In large numbers these molecules
attached themselves to the gold surface.

The scientists added nitrogen-containing (nitro) groups to the molecule’s center benzene ring. The resulting perturbed electron cloud made the asymmetric molecule twist when an electric field was set up by applying a voltage between the gold contacts. When the molecule twisted, current flow through the “molecular wire” was blocked. When the voltage was removed, the molecule adopted its original shape, allowing current to flow again.

Density
functional theory calculation of the molecular structure of three
benzenedithiols sandwiched between two gold surfaces.

Such a device, if fabricated on a large scale, could
be used as an ultrafast on-off switch, a key to creating ultrasmall, highly
dense computer circuits required to make computers fast and powerful enough
to mimic the human brain. Or the device could be used to make superior
computer memory elements. A charge can be stored on the nitro group to
prevent electrical conduction (binary 0), or the group can have no charge,
allowing conduction (binary 1). Such a molecular memory cell retains a
stored bit for nearly 10 minutes. By comparison, today’s silicon-dynamic,
random-access memories must be refreshed by an electrical current every
20 milliseconds. The new type of memory would save energy, allowing laptop
computer batteries to last 100 times longer.

Such a concept is being modeled computationally on
ORNL’s IBM supercomputer, dubbed Eagle, in a Laboratory Directed Research
and Development project. ORNL’s David Dean, Bill Butler (now at the University
of Alabama), Peter Cummings (an ORNL-UT Distinguished Scientist), David
Keffer (a University of Tennessee assistant professor), Predrag Krstic,
David Schultz, Mike Strayer, Jack Wells, and Xiaoguang Zhang are running
the calculations using a modified version of NWChem, a computational chemistry
code.

“Using ab initio methods, we modeled the self-assembly
and electrical conductivity of five benzenethiol (BT) and benzenedithiol
(BDT) molecules on a gold surface,” says Dean. In the example shown in
the illustration, two gold lattices are shown on the top and bottom. Three
BDT molecules are seen in the middle area. This particular configuration
has 70 atoms and 590 active electrons. A single calculation of this type
requires 46.67 hours on 80 nodes of Eagle, or 14,930 processor hours.
The single-particle wave functions resulting from this calculation will
be used in a conductance calculation to determine the current-voltage
characteristics of this molecular device.

The researchers have also performed preliminary molecular
simulations of self-assembled monolayers composed of BT molecules on the
[111] surface of gold. They included state-of-the-art force fields generated
through electronic structure calculations. Both molecular dynamics (MD)
and Monte Carlo (MC) simulations are being used. Gibbs ensemble MC simulation
is being used to establish the equilibrium between adsorbed monolayers
of BT and BDT with a low-density solution. MD is then being used to equilibrate
the structures thus found. “The structure of the adsorbed monolayers appears
to be consistent with available experimental results,” Dean says.

To produce a useful device, self-assembly must be combined with fabrication methods such as photolithography. The ORNL scientists will model how best to assemble these molecules and align them with the gold contacts to optimize electrical conductivity.

Device Design and Performance

A
carbon nanotube whose atoms are aligned with the carbon atoms of a
graphite sheet exhibits good current flow. By rotating the tube on
the sheet, tunable resistance results, making possible the creation
of a nano-rheostat that acts like a dimmer light switch. (Illustration
enhanced by LeJean Hardin)

Marco Buongiorno Nardelli, who holds a joint position
with ORNL and North Carolina State University (NCSU, one of UT-Battelle’s
core universities), has been exploring the feasibility of using carbon
nanotubes in nanoscale electronic devices. He is currently using Eagle
to run his own suite of codes simulating electron transport in carbon
nanotubes in contact with other materials.

In one project in which he provided computer modeling, experiments at the University of North Carolina (UNC) at Chapel Hill have shown that it is possible to build a nano-rheostat, similar to a dimmer light switch. In such a device, a carbon nanotubea cylinder resembling rolled-up chicken wire because its carbon atoms are arranged in a hexagonal configurationis placed on a sheet of graphite whose carbon atoms
also have a hexagonal arrangement.

“If you place the carbon cylinder on the graphite
sheet so that the carbon atoms of both are aligned, a current will flow
at the interface,” Buongiorno Nardelli says. “As you rotate the carbon
cylinder on the graphite sheet, changing the angle between the atoms in
the system, you get increased electrical resistance and reduced current
flow. As the atoms become aligned, you get low resistance and high current
flow.”

Computational simulations by Buongiorno Nardelli verified that the
interface between a carbon nanotube and graphite gives tunable resistance (as in a dimmer switch). His theoretical predictions on the feasibility of a nano-rheostat agreed with the UNC experimental results. The work was published in Science magazine in 2000.

Computational simulations suggest that electrical flow can be improved between a carbon nanotube and an aluminum contact by mechanically deforming the tube. (Illustration enhanced by LeJean Hardin)

If carbon nano-tubes are to be used as nanowires or
other components in nanoscale devices, electrons must flow between these
nanotubes and metal contacts in the device. In some experimental devices,
high resistance at the tube-contact interface can make the mechanism of
electron transfer quite inefficient. Buongiorno Nardelli and his NCSU
colleagues have used computer modeling to address the question of why
some nanodevices have better performance than others.

“In some devices,” Buongiorno Nardelli says, “electrons in the carbon nanotube stay in the tube and electrons in the aluminum stay in the metal. Our simulations suggest that contacts can be improved by mechanical deformations. For example, if a carbon nanotube sandwiched between two aluminum contacts is squeezed and deformed, new bonds form between the carbon and aluminum atoms, increasing electron flow at the tube-contact interface.”

The strength of carbon nanotubes is also of interest to Buongiorno Nardelli. Of all materials, carbon nanotubes have the highest tensile strength. They are 100 times stronger than steel but have one-sixth its weight. Scientists propose using carbon nanotubes as fibers in a polymer composite to form stronger structural materials for aircraft, spacecraft, and suspension bridges.

Computational simulations by Buongiorno Nardelli and his colleagues have shown that the geometrythe arrangement of the carbon hexagons along the nanotubeinfluences tube strength. “Our simulations,” he says, “predicted that whether a nanotube is brittle or ductile depends on the temperature at which it was deformed, the orientation of the hexagons with respect to the tube’s axis, and the amount it is stretched—that is, strain.”

Carbon nanotubes are very small, but simulations of their behavior in nanoscale electronic devices require a large amount of computer capacity.